This invention relates to an electronic musical instrument and, more particularly, to an electronic musical instrument wherein a tone is synthesized by generating partial tones of the tone and combining the partial tones.
Known in the art of a digital type electronic musical instrument employing the system of a tone partial synthesizing type is an instrument disclosed in the specification of U.S. Pat. No. 3,809,786. In the known instrument, amplitudes of W (quantity) harmonic contents of a tone are individually computed in a continuous time period t.sub.c =tx/w, where tx is a time interval for computing an amplitude value of one sample point of a musical tone waveform. The time period t.sub.c for computing each harmonic content is determined by dividing the time interval t.sub.x by the number W of the harmonic contents. Each harmonic content is computed in a specific one of the time periods t.sub.c fixedly corresponding to the harmonic content. That is, a first harmonic (fundamental) is computed in first time period (t.sub.c1), a second harmonic content in the second time period (t.sub.c2), . . . a sixteenth harmonic content in the sixteenth time period (t.sub.c16) respectively.
It will be observed from the above description that a time slot or a channel for generating a partial tone (hereinafter referred to as a partial when applicable) is always fixed. Accordingly, the sixteenth time period (time slot), for example, cannot be used for computing the second harmonic. Besides, if there is a harmonic which is of a higher order than sixteenth, a time slot (or a channel) exclusively allotted to that harmonic must be specially provided.
In some cases, a tone color can be formed with a relatively small number of partials, and in other cases, it needs a large number of partials. It is therefore necessary to keep as many partials as possible for possible necessity.
For meeting such requirements, the known electronic musical instruments employing the partial tone synthesizing system are designed to have as many time slots or channels (i.e. time periods for computing partials) as possible for generating partials. Since, however, each of the partial generating time slots is fixedly allotted to a specific one of the partials, there exist time slots which are not used at all in a case where a tone is synthesized only with a small number of partials or where an upper limit is imposed on a frequency component (harmonic frequency) of a produced tone by harmonic limiting. If, for instance, an 8-foot Tibia tone is to be produced by the aforementioned electronic musical instrument, the computation time period t.sub.c1 for computing the first harmonic and the computation time period t.sub.c3 for the third harmonic only are used and the remaining time periods for the second harmonic and the fourth through sixteenth harmonics are wasted. According to the prior art instrument, in a case where the frequency of a harmonic component of a musical tone is greater than the frequency of a sampling frequency at which the waveform of the musical tone is sampled, the harmonic is subject to "harmonic limiting" as disclosed in U.S. Pat. No. 3,882,751 resulting in non-use of time slots allotted for partials subject to harmonic limiting.
The prior art electronic musical instrument of the partial tone synthesizing type is disadvantageous also in that generation of plural tones on a time shared basis requires division of each time slot (i.e. time division channel) allotted for generation of each tone into sub-time slots for producing partials constituting the tone with a result that a maximum number of tones to be produced simultaneously cannot be sufficiently large for it is limited by the upper limit of the frequency of the system clock pulse. If the maximum number of tones to be produced simultaneously is represented by M, the number of partials prepared for synthesizing a tone by N, the period of time slots for producing the partials (i.e. the period of the system clock) by t.sub.c, and the sampling period by St, the sampling period St is St=M.multidot.N.multidot.t.sub.c.
The sampling period St cannot exceed a certain limit by reason of auditory sensation. The number N of the partials cannot be reduced beyond a certain limit if a satisfactory tone color is to be synthesized. There is also a certain limit in shortening the period t.sub.c of the system clock for technical reasons. For this reason, St and T.sub.c are fixed at certain limits and if a sufficient number of N is to be secured, the maximum number of tones M has to be reduced. In the prior art electronic musical instrument, the number of time slots for producing partials is fixed to N and, accordingly, the maximum number of tones to be produced simultaneously is also fixed to M which is rather an insufficiently small number once the system of the electronic musical instrument has been decided. Thus, a large number of time slots allotted for production of partials are left unused and these unused time slots are in no way utilized for increasing the maximum number of tones to be produced simultaneously.